A second linkage groups and the possibility of mitotic recombination in Schizosaccharomyces pombe.

Abstract

Introduction A!Iitotic recombination is one of a number of processes other than the sexual reproductive cycle, which may lead to new combinations of genetic material. The experimental value of mitotic recombination is considerable: it permits genetic analysis of organisms which have no sexual cycle or in which analysis through the sexual cycle is impracticable. In addition, certain problems in formal genetics, such as assignment of genes to cl~romosomes, location of cl~romosomes and recovery of reciprocal products of recombination can be solved more easily through the mitotic cycle, even when meiosis occurs and is amenable to study. Recombination at mitosis was first reported by Stern (1936) in Drosopbiln. However, his analysis was limited by the fact that the recombinant tissue could not be isolated and propagated. ~ i ~ n i f i c a n t ew developments did not come until the discovery of mitotic recombination in fungi (Pontccorvo and Roper, 1953). Here, the recombinant nuclei give rise to a clone that can be sampled and propagated at will. The recent progress in tissue culture should eventually make possible a considerable extension of the scope of the techniques which have been developed for the study of nlitotic recombination, since there is norhing at present to suggest that recombination is not a normal feature of mitosis. Chromosome pairing occasionally occurs between a particular pair of homologous chromosomes during mitosis; in fact, the low frequency of mitotic crossing over may well reflect the raritv of the event. The data from which somatic crossing over has been inferred are of the following type. When several chromosomes of the diploid are heterozygous at a number of loci, diploid segregants may be isolated which are homozygous at all loci distal to a point on one of the arms of a particular chromosome; i.e., they are homozygous diploid at these loci for the alleles derived from one parent. Howevcr, these recombinants are accompanied by a complementary group which are homozygous at the same loci for the alleles from the other parent. Both groups of recombinants normally remain heterozygous at all other loci. The possibility of mitotic crossing over within the regions of nd-7 and mating types in Scbizosncchnromy ces ponzbe was considered by Leupold ( 1958). Experiments with S. pombe (Ali, 1961) showed that crosses are difficult to establish in this organism. Providing mitotic crossing over occurs at a measurable rate in this organism it may become an invaluable tool for genetic studies.